Question

Which of the following microorganisms are used in Yogurt preparation?
• [(A).] Bacillus cereus 
• [(B).] Bacillus amyloliquefaciens 
• [(C).] Streptococcus thermophilus 
• [(D).] Lactobacillus bulgaricus 
Choose the correct answer from the options given below: 
 

• A. (A), (B) and (D) only.
• B. (C) and (D) only.
• C. (B), (C) and (D) only.
• D. (D) only.

Hint:

Remember that Yogurt fermentation is a classic example of protocooperation: a mutualistic, non-obligate relationship where both starter bacteria benefit from each other's metabolic products, resulting in a faster fermentation rate.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Yogurt is a fermented dairy product produced by fermenting milk with starter cultures of lactic acid bacteria (LAB).
These bacteria ferment lactose into lactic acid, which precipitates milk proteins to produce yogurt's characteristic texture and tart flavor.

Step 3: Detailed Explanation:
Traditional yogurt production relies on a symbiotic starter culture containing two specific lactic acid bacteria:
- Streptococcus thermophilus (C): A thermophilic, homofermentative bacterium that grows rapidly during the initial stages of fermentation.
It lowers the pH of the milk and consumes oxygen, creating the anaerobic conditions favored by the second bacterium.
- Lactobacillus delbrueckii subsp. bulgaricus (D) (often referred to simply as Lactobacillus bulgaricus): This rod-shaped bacterium is highly acid-tolerant.
It continues the fermentation process, lowering the pH further and producing aromatic compounds (such as acetaldehyde) that give yogurt its characteristic flavor.
These two species interact synergistically: L. bulgaricus breaks down milk proteins into amino acids that stimulate the growth of S. thermophilus, while S. thermophilus produces formic acid, which stimulates the growth of L. bulgaricus.
Let us evaluate the other options:
- Bacillus cereus (A): A spore-forming bacterium responsible for food poisoning; it is a milk contaminant, not a starter culture.
- Bacillus amyloliquefaciens (B): Used to produce industrial enzymes, not yogurt.
Therefore, (C) and (D) are the microorganisms used in yogurt preparation.

Step 4: Final Answer:
The correct combination is (C) and (D) only, matching Option (B).